engleski

Long-term changes in the northern Adriatic ecosystem related to anthropogenic eutrophication

The knowledge of the mechanisms driving long-term changes in the nutrient and primary production cycles represents a basic step to verify real eutrophication trends in shallow marine ecosystems, like the northern Adriatic open waters. In fact, this ecosystem appears to be especially sensitive to seasonal and long-term variations of the anthropogenic nutrient load (primarily the Po River discharges), which impact can be significantly modified by changes in the oceanographic conditions, due to climatic fluctuations. To verify this assumption, the data for several parameters related to eutrophication (salinity, temperature, transparency, chlorophyll a, primary productions rate, nutrients, water transparency, dissolved oxygen), collected in the period 1966-1995 in open waters, were analysed and compared with changes of the Po River flow rate. The data were grouped in typical seasons, periods and subareas, characterized by different dominant relevant processes. The changes of mean values for salinity and temperature were well correlated with the Po flow rates in all periods, except during the late eighties. In these years the surface salinity was generally lower and the temperature was higher than expected from the flow rate values. This departure was explained by unusual hydrometeorological conditions, characterized by a calm sea and sunny weather, due to long periods of high barometric pressure. In these conditions the freshwater mixing was limited to a thinner surface layer, in which heat accumulation was favoured. The long term-changes of the nutrient concentrations were even less correlated with the Po flow rates, compared to salinity and temperature. The mean orthophosphate concentration of the surface layer in the season February-April of the late eighties appeared to be lower, and that of the total inorganic nitrogen (TIN) higher than expected from the pattern of the Po flow rate. These deviations can be explained by long-term changes of the river nutrient composition. In relation to this, when compared at the same salinity (i.e. same dilution degree, independently on the freshwater discharge rate), the chlorophyll a concentrations and the photosynthetic activities (estimated by seawater incubation at constant light with 14C-bicarbonate) were higher in periods of higher river orthophosphate concentrations, but not of TIN concentrations. Furthermore, the data analysis suggested that more favourable hydrometeorological conditions for phytoplankton growth in spring and summer occurred during the late eighties compared to other periods with similar phosphorus availabilities (i.e. the seventies). Intense surface phytoplankton blooms, but localised off the Po Delta, were induced in unusually long situations of calm sea and sunny weather. Long-term changes of the primary production parameters in the autumn and winter were not related to the nutrient composition of the Po waters. Probably, the primary production in these seasons is mainly controlled by nutrient recycling within the marine ecosystem. The bottom oxygen saturation during summer and autumn of the late eighties was lower than expected from the Po flow rate, especially if the decrease of the phosphorus load is considered. In fact, the near-anoxia events (and the autumn1989 anoxia) in the bottom layer of large areas probably occurred as a consequence of a delayed reactivation of the autumn vertical and horizontal water circulation, due to unusual meteorological conditions. This is also confirmed by the fact that similar events were not observed in the successive period (1990-95), characterized by different meteorological and hydrological cycles. The analysis of the long-term change of the Po River flow rate was expanded to the entire measurement period (since 1917). It was concluded that during the period 1969-1992 the flow rate in spring was on average higher than in the previous period, and that the autumn peak was anticipated from November to October. These changes might have had some influence on the northern Adriatic ecosystem. Moreover, particularly frequent was the occurrence of a secondary peak flow in March during the seventies. It was concluded that significant fluctuations of primary production in the open northern Adriatic occurred since the seventies, caused by variations of the nutrient load and oceanographic conditions. A possible reduction of the eutrophication pressure on the northern Adriatic ecosystem should be confirmed extending the monitoring activities, also considering that additional restoration measures are planned for the near future to reduce the nutrient load within the Po watershed.

Adriatic sea ; Eutrophication ; Po river ; Primary production

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